The present invention relates to an apparatus for compensating a thermal asperity in the data channel of, for example, a hard disk drive which has an MR (Magenetoresitive) head used as a read head.
In recent years, attention is paid to hard disk drives (HDDs) with a read/write head which comprises an MR head used as the read head and an inductive head used as the write head. The read/write head of this type is advantageous in terms of data-recording and data-reproducing efficiency because each head can be modified for higher operating efficiency. The MR head is a component whose resistance changes with the intensity of the magnetic flux emanating from a magnetic disk. The changes in the resistance of the MR head are detected by the data-reproducing unit incorporated in the HDD. The data-reproducing unit converts the resistance changes to signal voltages. The data channel (read channel) of the HDD processes the signal voltages, whereby data is reproduced from the disk.
In the HDD, both the read head (MR head) and the write head (inductive head) are mounted on a head slider. The head slider moves, floating above the disk, so that the read head may read data from the disk and the write head may record data on the disk. More precisely, the head slider moves as if flying over the disk at an extremely small flying height of, for example, about 50 nm. Recently the flying height is reduced to increase the S/N ratio of data signals in proportion to the density at which data is recorded on the disk. The less the flying height, the greater the possibility that the lower surface of the head slider contacts the disk while moving over the disk.
Mounted on the lower surface of the head slider is the MR head (read head). If the MR head collides with the disk, an impact energy will be generated. The impact energy raises the temperature of the MR head, greatly changing the resistance of the MR head. As a consequence, the data signal (read signal) the MR head produces has its amplitude (i.e., DC level) changed very much as is shown in FIG. 11A.
The phenomenon that the resistance of the MR head greatly changes with the temperature of the MR head is known as "thermal asperity (TA)." The thermal asperity (TA) results in changes in the DC level of the data signal (i.e. thermal disturbance). To be more specific, the DC level (amplitude) of the data signal sharply increases as the temperature of the MR head rises and exponentially decreases as the heat is radiated from the MR head, lowering the temperature thereof.
Once the amplitude of the data signal changes due to the thermal asperity, it cannot be correctly processed (or demodulated) in the read channel of the HDD until it regains the normal level. This imposes an adverse influence on the AGC (Automatic Gain Control) amplifier provided in the read channel. The AGC amplifier is a feedback circuit designed to maintain the data signal at a constant level. Thus the thermal asperity renders it difficult for the read channel to reproduce data from the disk. In other words, the thermal asperity is a cause for an increase of the read-error rate.
In the conventional HDD, the disk controller (HDC) provided for control of the transfer of data has the function of correct read errors. The disk controller can correct burst errors caused by a thermal asperity (TA) which terminates in a relatively short time. A TA-compensating method has been developed in which a high-pass filter (HPF) is used in the read channel to terminate the change in the signal DC level (thermal disturbance) quickly. This TA-compensating method can terminate the change in the signal DC level quickly within a relatively short time as seen from FIG. 11B.
FIG. 10 shows a read channel 4 for use in an HDD, which processes digital signals of PRML (Partial Response Maximum Likelihood) type. In the read channel 4, the HPF 23 for compensating a thermal asperity (TA) is connected to the input of the AGC amplifier 25, and the TA detector 20 is connected to the input of the HPF 23. The TA detector 20 receives a read signal supplied from the MR head 2 and amplified by a head amplifier 3, for detecting the change in the DC level (i.e., thermal disturbance) of the read signal.
The TA detector 20 detects the change in the DC level of the read signal (i.e., the output signal of the head amplifier 3), on the basis of the reference level (known as "slice level) set by a device provided outside the read channel, i.e., the CPU incorporated in the HDD. The TA detector 20 generates a signal representing the change in the DC level of the read signal.
In the read channel 4, the output signal of the TA detector 20 is supplied to the delay circuit 22. The delay circuit 22 generates a hold signal HOLD from the output signal of the TA detector 20. The signal HOLD has a pulse width which corresponds to the sum of the TA-detecting time and the prescribed delay time of the circuit 22. The signal HOLD is supplied to the AGC circuit 30 and the read PLL 31, prohibiting both the AGC circuit 30 and the read PLL 31 from receiving data signals for the period corresponding to the pulse width of the signal HOLD. As long as the AGC circuit 30 and the read PLL 31 are prohibited from receiving data signals, any data signal that has an abnormal amplitude for several bits is not input to the AGC circuit 30 or the read PLL 31, even if its DC level change is reduced by the HPF 23.
The HPF 23 is a programmable filter having parameters (such as the cutoff frequency fc) each of which can be set to different values by the CPU. The read channel 4 further comprises a low-pass filter (LPF) 26, an A/D converter 27, a digital equalizer 28, a viterbi decoder (detector) 29, and a decoder 32. The viterbi decoder 29 performs data-decoding (data-detection) of ML (Maximum Likelihood) type. The decoder 32 decodes the read data to write data (RD), which is supplied to the HDC 5. The read channel 4 is activated by a read gate signal (RG) supplied from the HDC 5. The read PLL 31 supplies a timing clock signal to the A/D converter 27 and the viterbi decoder 29. The read PLL 31 also supplies a read clock signal (i.e., channel clock signal) RCLK to the HDC 5. The AGC circuit 30 outputs a control signal for controlling the the gain of the AGC amplifier 25, which is provided to maintain the read signal at a constant level.
To terminate the change in the signal DC level within the shortest time possible, it is desired that the cutoff frequency fc of the HPH 23 be as high as possible. However, the higher the cutoff frequency fc of the HPF 23, the more the phase characteristic of the waveform of the read signal is deteriorated in the low-frequency band. Hence, the efficiency at which the HPF 23 compensates the thermal asperity (TA) needs to be traded off with the efficiency at which the read channel reproduce data from the disk.
As can be understood from FIGS. 11A and 11B, it is assumed that the DC level of the read signal changes about three times as much as it does in normal conditions. How much the DC level changes depends upon how much the temperature of the MR head rises due to the collision of the MR head with the disk. The time required to cool the MR head to the initial temperature varies with the rotational speed of the disk and the temperature outside the HDD. The number of bits making a read-burst error depends on the density at which data is recorded on the disk. In view of these points, it is desired that the cutoff frequency fc of the HPF 23 should be a variable one.
The change in the DC level of the read signal can be reduced by using the HPF 23 to compensate the thermal asperity (TA), as is seen from the waveform 72 shown in FIG. 7A which is the waveform of the output of the LPF 26. In FIG. 7A, the waveform 70 pertains to the output of the head amplifier 3, and the waveform 71 to the output the LPF 26 might generate if the HPF 23 were not provided. Were the thermal asperity (TA) not compensated for, the LPF 26 would generate an output having the waveform 71, not an output having the waveform 72. For convenience, only the DC components of the outputs of the head amplifier 3, HPF 23 and LPF 26 are shown in FIG. 7A and it is assumed that the DC levels of these outputs change about three times as much as they do in normal conditions and that the cutoff frequency fc of the HPF 23 is about 0.03 times the frequency of the channel clock signal RCLK of the read channel 4. As clearly seen from the waveforms 71 and 72, the change in the DC level of the read signal decreases within a short time since the HPF 23 compensates the thermal asperity (TA).
FIG. 7B is an enlarged graph, showing a part of the graph of FIG. 7A. As shown in FIG. 7B, the output signal 72 of the HPH 23, i.e., the TA-compensated read signal remains at a negative DC level for a relatively long time. This phenomenon, known as "DC undershoot (US)," will increase the read-error rate in the read channel 4, particularly in the viterbi decoder 29. Although the HPH 23 compensates the thermal asperity (TA) to suppress the change in the DC level of the read signal, it causes an DC undershoot, inevitably increasing the read-error rate in the read channel 4.